Because of the great amount of pressure thousands of feet beneath the earth's surface, oil tries to move to areas of less pressure. If it is allowed, it will move upwards until it is above ground. This is what happens at oil seeps (once common in Pennsylvania, California, Texas and Louisiana, among other places). While these seeps tell us there is oil below ground, it also tells us that much oil has already escaped, and it may mean that there isn't much left to find. Unlike a reservoir rock, which acts like a sponge, trap rocks act like walls and ceilings, and will not allow fluids to move through. The most common trap rock is shale, which, when compared to many sandstones, has very little room inside for fluids (oil, for example) to move through it.

Though trap rocks block oil from moving through them, they don't always block oil from moving around them. For a trap rock to do its job, we need some kind of geologic trap.

This is what a trap rock would look like through a magnifying lens. The yellow objects represent clay particles that are packed together. Notice the very small amount of space between the clay particles. A similar situation would be if you took individual playing cards and laid them flat on top of one another - there would be very little space in between. Because there is no space between clay particles, oil will not move through this rock - instead, it will be blocked. Compare this rock to a reservoir rock .

Suggested Exercise

Take 2 pieces (10" x 10") of fine mesh screen, and put a bucket under both. On one screen, smear some mud (as pure as possible). On another, smear sand. While still wet, take a glass of water. Pour half of the water on the screen with sand, and watch it drip through. Do the same with the muddy screen.

Why the difference?

Which rock (shale - made out of hardened clay, or sandstone - made out of hardened and cemented sand) do you suppose would "suck up" oil? Which rock do you suppose would block oil from moving through (thereby "trapping" it into place)?